One conventional microprocessor memory interface includes an address bus, a data bus, and a control bus comprised of a plurality of control signals (e.g., read/write). The number of address signal lines required depends upon the size of the memory (i.e., the number of addressable memory locations). The number of data signal lines required depends upon the type of microprocessor used. Conventional data bus widths are typically n(8) bits, where n is an integer equal to or greater than one. The control bus has a number of lines required for at least selecting, reading, or writing the external memory.
By example, for an 8 Mbit memory that is organized as 1024 kbits.times.8 bits, 20 address lines and eight data lines are required. A memory access is performed by first applying the address of a memory location, and then asserting one or more control lines to perform the desired memory read or write access. For a memory write operation it is necessary to also drive the date bus with the data to be written. If the 8 Mbit memory is instead organized as 524 k.times.16 bits, then 19 address lines and 16 data lines are required in order to access the entire memory.
It is also known in the microprocessor field to employ a multiplexed address/data bus, wherein at least some of the address signal lines also function as data lines. For example, during a first part of a memory access cycle the multiplexed lines convey address information to the memory, while in a later part of the cycle these same lines convey data that is written to or that is read from the memory. For this type of architecture it is known to provide an address valid or similar control signal line for indicating to external circuitry when the multiplexed address/data lines are conveying address information as opposed to data. This control signal line is typically used to latch the address information into an external latching device so that the address information remains stable after the multiplexed address/data bus switches from the address to the data mode of operation.
The power consumption of a digital integrated circuit is a function of a number of parameters, including the number of external output signal lines that are driven and the number of state changes at each output. These factors become increasingly important when one integrates a microprocessor and external memory into a battery operated device, such as a handheld wireless telephone, a cellular telephone, or a personal communicator. In such a device it is desirable to reduce the overall power consumption as much as possible in order to increase the time between required rechargings of the battery.